Many petrochemical products are produced by the oxidation of an appropriate hydrocarbon in the vapor phase over a suitable catalyst. For example, maleic anhydride is produced commercially by the vapor phase oxidation of benzene or straight-chain C.sub.4 hydrocarbons (hydrocarbons containing four carbon atoms), such as n-butane, butene or butadiene, with oxygen over a vanadium-phosphorus oxide catalyst. Similarly, unsaturated nitriles are produced by the ammoxidation of a saturated or olefinically unsaturated hydrocarbon with oxygen in the presence of ammonia and an appropriate catalyst; alkylene oxides are produced by the oxidation of lower alkanes or alkenes with oxygen in the presence of an appropriate catalyst; and unsaturated chlorinated hydrocarbons are produced by the oxidation of lower alkanes or alkenes with oxygen in the presence of an appropriate catalyst. Air is generally used as the source of the oxygen because of its low cost and ready availability. The reaction can be carried out in any suitable reactor, such as a fixed, fluidized or transport bed reactor, and it produces the petrochemical product, and generally carbon monoxide (CO), carbon dioxide (CO.sub.2), water, and smaller amounts of other partially oxidized by-products. The reaction equipment train generally consists of a reactor, in which the desired product is produced, a petrochemical recovery unit such as a scrubber, in which the product is recovered from the reactor effluent gases by means of water or other solvent for the desired product, and means for further treating the scrubbed effluent gases.
In the past it was common to practice the above-described processes on a single pass basis with the conversion of hydrocarbon being maximized. This often resulted in a low overall efficiency, since the selectivity to the desired product may have been below the maximum. Consequently, the product-depleted effluent gas contained, in addition to unreacted hydrocarbon, considerable amounts of CO and CO.sub.2. These products were usually incinerated, so that the only return realized from them was heat value. In later processes a portion of the product-depleted effluent gas was recycled, the conversion of the hydrocarbon feedstock was lowered and the selectivity of hydrocarbon conversion to desired products was maximized. The remainder of the effluent was purged from the system to prevent the build-up of CO, CO.sub.2 and nitrogen (introduced into the system when air is used as the source of oxygen). These improvements resulted in a reduced "per pass" conversion but the overall efficiency of the process was increased.
Federal Republic of Germany (FRG) Patent Application Disclosure 25 44 972 discloses a maleic anhydride manufacturing process in which the reactor feed comprises C.sub.4 hydrocarbons, air, CO and CO.sub.2. In the process of this patent maleic anhydride is recovered from the reactor effluent gas stream and a portion of the remaining stream is recycled. This patent also teaches recovering butane by temperature swing adsorption from the non-recycled gas stream and recycling the recovered butane to the reactor.
A major problem associated with the gas phase production of a petrochemical by the oxidation of hydrocarbons with oxygen is that since the reaction is carried out at elevated temperatures, there is an ever-present danger of a fire or an explosion in the reactor, or the equipment or pipelines associated with the reactor, as a result of the decomposition of unreacted hydrocarbons. The propensity of the hydrocarbons to decompose is enhanced by the presence of catalyst, and the tendency toward decomposition is particularly enhanced in fluidized bed or transport bed reactors. Accordingly, efforts are constantly made to maintain conditions in the reactor and associated equipment such that the mixture remains outside of the flammability range, or at least out of the autoignition range.
U.S. Pat. No. 3,904,652 teaches a gas phase maleic anhydride manufacturing process in which oxygen is used as the oxidizing gas and an inert gas, such as nitrogen, argon, helium or a lower hydrocarbon is fed into a fixed bed reactor with the n-butane and oxygen, the inert gas serving as a diluent to reduce the concentrations of oxygen and butane in the reactor to below the point at which they form a flammable mixture. In the disclosed process, a portion of the gaseous effluent, which contains, in addition to butane, carbon monoxide, carbon dioxide and the inert gas, is recycled.
U.S. Pat. No 4,352,755 discloses a recycle process for the vapor phase manufacture of maleic anhydride by reacting a straight-chain C.sub.4 hydrocarbon with oxygen in the presence of CO.sub.2. In the process disclosed in this patent the gaseous mixture may contain up to 30 volume percent of carbon dioxide as the inert diluent and contains at least 25 volume percent C.sub.4 hydrocarbon. This patent states that at most 2% v/v and more preferably at most 1% v/v of carbon monoxide is present in the oxidation stage. In the process of this patent, the presence of large amounts of C.sub.4 hydrocarbon can render the gas mixture in the system flammable, especially in the region of the reactor outlet.
As is well known, under a given set of conditions, including composition and pressure, the flammability and autoignitability of a gaseous hydrocarbon-oxygen mixture is dependent upon, inter alia, the temperature of the gaseous mixture. At low temperatures, the gaseous mixture may have a relatively small flammability range, but as the temperature of the mixture rises, its flammability range increases. For instance, at low temperatures (lower than about 300.degree. C.) the mixture may not be autoignitable. However, as the temperature rises, a point is eventually reached at which the mixture becomes autoignitable. When this point is reached, the mixture will ignite and burn or explode, which event can result in damage to equipment and serious injury or death to persons in the vicinity of the fire or explosion.
A feed mixture entering the oxidation reactor of a vapor phase petrochemical manufacturing plant may have a composition that, at higher temperatures, would render it self-ignitable. The reaction occurring in the reactor upon contact of the feed mixture with the catalyst often raises the temperature of the mixture to the point at which it would ordinarily ignite, however the mixture usually does not ignite in the reactor, apparently because the catalyst suppresses ignition of the mixture. On the other hand, since very little, if any, catalyst is entrained in the hot product gas stream leaving the reactor, it often happens that the gas stream leaving the reactor is subject to self-ignition. To prevent such an occurrence, the mixture leaving the reactor is conventionally rapidly cooled by heat exchange to a temperature below its autoignition point. It sometimes happens, however, that the reactor exit heat exchanger fails to cool the product gas stream rapidly enough to prevent it from igniting, and a devastating and costly fire occurs.
Because of the considerable danger of injury to personnel and damage to equipment, efforts to find new and improved ways to eliminate or reduce the hazard of fire or explosion in chemical plants are continuously made. The present invention provides a method and apparatus for reducing the likelihood of a fire or explosion in a plant for the vapor phase manufacture of petrochemicals.